Normalization. Normalization. Normalization. Data Redundancy

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1 Normalization Normalization o Main objective in developing a logical data model for relational database systems is to create an accurate representation of the data, its relationships, and constraints. o To achieve this objective, we must identify a suitable set of relations => The Normalization Technique o Developed by E.F. Codd (1972). Normalization - 1 Normalization - 2 Normalization o Often performed as a series of tests on a relation to determine whether it satisfies or violates the requirements of a given normal form. o 4 most commonly used normal forms are first (1NF), second (2NF), third (3NF) and Boyce-Codd (BCNF) normal forms. Data Redundancy o Major aim of relational database design is to group attributes into relations to minimize data redundancy and reduce file storage space required by base relations. o Based on functional dependencies among the attributes of a relation. o A relation can be normalized to a specific form to prevent the possible occurrence of update anomalies. Normalization - 3 Normalization - 4

2 Data Redundancy Update Anomalies o Relations that contain redundant information may potentially suffer from update anomalies. o Staff_Branch relation has redundant data; the details of a branch are repeated for every member of staff. o Types of update anomalies include Insertion Deletion (eg. in the Staff_Branch relation, for a particular branch, removal of the last member of staff => lost of branch information) Modification Normalization - 5 Normalization - 6 Functional Dependency Functional Dependency: o Main concept associated with normalization. Functional Dependency Diagrammatic representation: o Functional Dependency Describes the relationship between attributes in a relation. For example, if A and B are attributes of relation R, B is functionally dependent on A (denoted A -> B), if each value of A in R is associated with exactly one value of B in R. Normalization - 7 Normalization - 8

3 Functional Dependency 3 Types of Functional Dependencies Full Dependency o In a relation, the attribute(s) B is fully functional dependent on A if B is functionally dependent on A, but not on any proper subset of A. Partial Dependency o If there is some attribute that can be removed from A and the dependency still holds. Eg. Staff_No, Sname -> Branch_No Normalization - 9 Normalization Types of Functional Dependencies Transitive Dependency o In a relation, if attribute(s) A->B and B->C, then C is transitively dependent on A via B (provided that A is not functionally dependent on B or C) Eg. Staff_No->Branch_No and Branch_No->BAddress The Process of Normalization o Normalization: Formal technique for analyzing a relation based on its primary key and the functional dependencies between the attributes of that relation. o Often executed as a series of steps. Each step corresponds to a specific normal form, which has known properties. o As normalization proceeds, the relations become progressively more restricted (stronger) in format and also less vulnerable to update anomalies. Normalization - 11 Normalization - 12

4 Relationship Between Normal Forms Unnormalized Normal Form o Unnormalized Normal Form (UNF) A table that contains one or more repeating groups. To create an unnormalized table Transform the data from the information source (e.g. form) into table format with columns and rows. Normalization - 13 Normalization - 14 Example - A Report Example - UNF Normalization - 15 Normalization - 16

5 First Normal Form 1NF - A relation in which the intersection of each row and column contains one and only one value. Converting from UNF to 1NF: o Select attribute(s) to act as the key. o Identify the repeating group(s) in the unnormalized table which repeats for the key attribute(s). o Remove the repeating group by Entering data into empty columns of rows which contain the repeating data. Or by Placing the repeating data along with a copy of the original key attribute(s) into a separate relation. Normalization - 17 Example - Normalization UNF to 1NF Normalization - 18 Example - UNF to 1NF (Alternative) Second Normal Form Based on the concept of full functional dependency. A 2NF relation is in 1NF and every non-primary-key attribute is fully functionally dependent on the primary key. Converting from 1NF to 2NF: o Identify the primary key for the 1NF relation. o Identify the functional dependencies in the relation. Normalization - 19 o If partial dependencies exist on the primary key remove them by placing then in a new relation along with a copy of their determinant. Normalization - 20

6 FDs for Customer_Rental Relation Customer_Rental to 2NF Relations Primary key: Customer_No + Property_No Full Functional Dependency: (Customer_No+Property_No)->(RentStart, RentFinish) Partial Dependency: (Customer_No+Property_No)->Cname (Customer_No+Property_No)->(Paddress, Rent, Owner_No, Oname) Normalization - 21 Normalization - 22 Third Normal Form (3NF) Property_Owner to 3NF Relations Based on the concept of transitive dependency. A relation that is in 1NF and 2NF and in which no non-primary-key attribute is transitively dependent on the primary key. Converting from 2NF to 3NF: o Identify the primary key in the 2NF relation. Transitive Dependency: (Customer_No+Property_No)->Owner_No Owner_No ->OName o Identify functional dependencies in the relation. o If transitive dependencies exist on the primary key remove them by placing them in a new relation along with a copy of their dominant. Normalization - 23 Normalization - 24

7 Process of Decomposition Summary of 3NF Relations Normalization - 25 Normalization - 26 Boyce-Codd Normal Form (BCNF) o Based on functional dependencies that takes into account all candidate keys in a relation. o For a relation with only one candidate key, 3NF and BCNF are equivalent. o A relation is in BCNF, if and only if every determinant is a candidate key. o Violation of BCNF may occur in a relation that contains 2 (or more) composite keys which overlap and share at least 1 attribute 3NF to BCNF o Identify all candidate keys in the relation. o Identify all functional dependencies in the relation. o If functional dependencies exists in the relation where their determinants are not candidate keys for the relation, remove the functional dependencies by placing them in a new relation along with a copy of their determinant. Normalization - 27 Normalization - 28

8 Example - 3NF to BCNF Relations Example - 3NF to BCNF Relations Everyday a staff is allocated a room only. Staff may share rooms. Clients maybe interviewed again. (Client_No, Interview_Date) -> (Interview_Time, Staff_No, Room_No) (Staff_No, Interview_Date, Interview_Time) -> Client_No (Room_No, Interview_date, Interview_Time) -> Staff_No, Client_No (Staff_No, Interview_Date) -> Room_No Normalization - 29 Normalization - 30 The Process of Normalization up to BCNF Example UNF Remove repeating groups 1NF Remove partial dependencies 2NF Remove transitive dependencies 3NF Remove remaining anomalies from functional dependencies BCNF Normalization - 31 Normalization - 32

9 Example - UNF to 1NF Relation Example - 1NF to 2NF 1NF: Property_Inspection (Property_No, IDate, ITime, Paddress, Comments, Staff_No, Sname, Car_Reg) Full Functional Dependency: (Property_No+IDate)->(ITime, Comments, Staff_No, Sname, Car_Reg) Partial Dependency: (Property_No+IDate)->(PAddress) => 2NF: Prop (Property_No, Paddress) Prop_Inspection (Property_No, IDate, ITime, Comments, Staff_No, Sname, Car_Reg) Normalization - 33 Normalization - 34 Example - 2NF to 3NF Transitive Dependency in Prop_Inspect: (Property_No+IDate) -> Staff_No Staff_No -> Sname => 3NF: Staff (Staff_No, Sname) Prop_Inspection (Property_No, IDate, ITime, Comments, Staff_No, Car_Reg) Example - 3NF to BCNF Prop (Property_No,Paddress) Staff (Staff_No, Sname) Prop_Inspection (Property_No, IDate, ITime, Comments, Staff_No, Car_Reg) Prop and Staff are already in BCNF. FDs of Prop_Inspect: o (Property_No, IDate)->(ITime, Comments, Staff_No, Car_Reg) o (Staff_No, Idate) -> Car_Reg o (Car_Reg, Idate, ITime) -> (Property_No, Comments, Staff_No) o (Staff_No, Idate, ITime) -> (Property_No, Comments) Normalization - 35 Normalization - 36

10 Example BCNF Process of Decomposition Prop (Property_No,Paddress) Staff (Staff_No, Sname) Inspection (Property_No, IDate, ITime, Comments, Staff_No) Staff_Car (Staff_No, IDate, Car_Reg) Normalization - 37 Normalization - 38